page_title: Red giant
section_title: Evolution
hierarchical_section_title: Red giant / Evolution
caption_attribution_description: English: This image tracks the life of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant star on the right. On the left the star is seen as a protostar, embedded within a dusty disc of material as it forms. It later becomes a star like our Sun. After spending the majority of its life in this stage, the star's core begins to gradually heat up, the star expands and becomes redder until it transforms into a red giant. Following this stage, the star will push its outer layers into the surrounding space to form an object known as a planetary nebula, while the core of the star itself will cool into a small, dense remnant called a white dwarf star. Marked on the lower timeline are where our Sun and solar twins 18 Sco and HIP 102152 are in this life cycle. The Sun is 4.6 billion years old and 18 Sco is 2.9 billion years old, while the oldest solar twin is some 8.2 billion years old — the oldest solar twin ever identified. By studying HIP 102152, we can get a glimpse of what the future holds for our Sun. This image is illustrative; the ages, sizes, and colours are approximate (not to scale). The protostar stage, on the far left of this image, can be some 2000 times larger than our Sun. The red giant stage, on the far right of this image, can be some 100 times larger than the Sun. Deitsch: Diese Grafik zeigt das Leben eines sonnenähnlichen Sterns von dessen Geburt auf der linken Seite bis zu seiner Entwicklung zu einem Roten Riesen auf der rechten Seite. Links ist der Stern als Protostern zu sehen, während seiner Entstehung eingehüllt in eine staubige Materiescheibe. Später wird er zu einem Stern ähnlich unserer Sonne. Nachdem er den größten Teil seines Lebens in diesem Entwicklungsstadium verbracht hat, beginnt der Kern des Sterns sich nach und nach aufzuheizen. Der Stern dehnt sich aus und wird röter, bis er sich in einen Roten Riesen verwandelt hat. Nach diesem Stadium wird der Stern seine äußere Hülle in seine Umgebung abstoßen und zu einem Objekt werden, das unter dem Namen Planetarischer Nebel bekannt ist. Der Kern des Sterns selbst wird abkühlen und zu einem kleinen, dichten Überbleibsel werden, einem Weißen Zwerg. Die Grafik ist nur schematisch; Alter, Größen und Farben sind geschätzt und nicht maßstabsgetreu dargestellt. Der Protostern, weit links in der Grafik, kann etwa 2000 mal größer sein als unsere Sonne. Der Rote Riese, weit rechts in der Grafik, kann etwa 100 mal größer sein als die Sonne.
context_page_description: A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K or lower. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars.
  Red giants differ in a way by which they generate energy:
  most common red giants are stars on the red-giant branch that are still fusing hydrogen into helium in a shell surrounding an inert helium core
  red-clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process
  asymptotic-giant-branch stars with a helium burning shell outside a degenerate carbon–oxygen core, and a hydrogen-burning shell just beyond that.
  Many of the well-known bright stars are red giants, because they are luminous and moderately common. The K0 RGB star Arcturus is 36 light-years away, and Gamma Crucis is the nearest M-class giant at 88 light-years' distance.
context_section_description: Red giants are evolved from main-sequence stars with masses in the range from about 0.3 M☉ to around 8 M☉. When a star initially forms from a collapsing molecular cloud in the interstellar medium, it contains primarily hydrogen and helium, with trace amounts of "metals" (in stellar structure, this simply refers to any element that is not hydrogen or helium i.e. atomic number greater than 2). These elements are all uniformly mixed throughout the star. The star reaches the main sequence when the core reaches a temperature high enough to begin fusing hydrogen (a few million kelvin) and establishes hydrostatic equilibrium. Over its main sequence life, the star slowly converts the hydrogen in the core into helium; its main-sequence life ends when nearly all the hydrogen in the core has been fused. For the Sun, the main-sequence lifetime is approximately 10 billion years. More-massive stars burn disproportionately faster and so have a shorter lifetime than less massive stars.
  When the star exhausts the hydrogen fuel in its core, nuclear reactions can no longer continue and so the core begins to contract due to its own gravity. This brings additional hydrogen into a zone where the temperature and pressure are sufficient to cause fusion to resume in a shell around the core. The hydrogen-burning shell results in a situation that has been described as the mirror principle; when the core within the shell contracts, the layers of the star outside the shell must expand. The detailed physical processes that cause this are complex, but the behaviour is necessary to satisfy simultaneous conservation of gravitational and thermal energy in a star with the shell structure. The core contracts and heats up due to the lack of fusion, and so the outer layers of the star expand greatly, absorbing most of the extra energy from shell fusion. This process of cooling and expanding is the subgiant star. When the envelope of the star cools sufficiently it becomes convective, the star stops expanding, its luminosity starts to increase, and the star is ascending the red-giant branch of the Hertzsprung–Russell (H–R) diagram.
  The evolutionary path the star takes as it moves along the red-giant branch depends on the mass of the star. For the Sun and stars of less than about 2 M☉ the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further. Once the core is degenerate, it will continue to heat until it reaches a temperature of roughly 10⁸ K, hot enough to begin fusing helium to carbon via the triple-alpha process. Once the degenerate core reaches this temperature, the entire core will begin helium fusion nearly simultaneously in a so-called helium flash. In more-massive stars, the collapsing core will reach 10⁸ K before it is dense enough to be degenerate, so helium fusion will begin much more smoothly, and produce no helium flash. The core helium fusing phase of a star's life is called the horizontal branch in metal-poor stars, so named because these stars lie on a nearly horizontal line in the H–R diagram of many star clusters. Metal-rich helium-fusing stars instead lie on the so-called red clump in the H–R diagram.
  An analogous process occurs when the central helium is exhausted and the star collapses once again, causing helium in a shell to begin fusing. At the same time hydrogen may begin fusion in a shell just outside the burning helium shell. This puts the star onto the asymptotic giant branch, a second red-giant phase. The helium fusion results in the build up of a carbon–oxygen core. A star below about 8 M☉ will never start fusion in its degenerate carbon–oxygen core. Instead, at the end of the asymptotic-giant-branch phase the star will eject its outer layers, forming a planetary nebula with the core of the star exposed, ultimately becoming a white dwarf. The ejection of the outer mass and the creation of a planetary nebula finally ends the red-giant phase of the star's evolution. The red-giant phase typically lasts only around a billion years in total for a solar mass star, almost all of which is sp
caption: This image tracks the life of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant on the right after billions of years
exif: {"Image ImageDescription": "This image tracks the life of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant star on the right. On the left the star is seen as a protostar, embedded within a dusty disc of material as it forms. It later becomes a star like our Sun. After spending the majority of its life in this stage, the star's core begins to gradually heat up, the star expands and becomes redder until it transforms into a red giant. Following this stage, the star will push its outer layers into the surrounding space to form an object known as a planetary nebula, while the core of the star itself will cool into a small, dense remnant called a white dwarf star. Marked on the lower timeline are where our Sun and solar twins 18 Sco and HIP 102152 are in this life cycle. The Sun is 4.6 billion years old and 18 Sco is 2.9 billion years old, while the oldest solar twin is some 8.2 billion years old \u2014 \u00a0the oldest solar twin ever identified. By studying HIP 102152, we can get a glimpse of what the future holds for our Sun. This image is illustrative; the ages, sizes, and colours are approximate (not to scale). The protostar stage, on the far left of this image, can be some 2000  times larger than our Sun. The red giant stage, on the far right of this  image, can be some 100 times larger than the Sun."}